The present invention concerns a method for improving growth characteristics of plants by modulating activity of a mutant CDKA kinase or a homologue thereof in a plant and/or modulating expression of a nucleic acid encoding such mutant CDKA. One such method comprises introducing into a plant a mutant CDKA nucleic acid molecule or mutant functional variant thereof. The invention also provides an isolated CKA mutant protein and nucleic acids encoding such protein. The invention furthermore relates to transgenic plants having improved growth characteristics, which plants have modulated expression of a nucleic acid encoding a mutant CDKA kinase. The present invention also concerns constructs useful in the methods of the invention.